1. Field of the Invention
The present invention relates to a method of error modification for video data of the Y, C.sub.R, and C.sub.B component system consisting of a luminance signal and two color different signals, and more particularly is directed to a method of modifying residual error data that have been uncorrectable in an error correction process.
2. Description of the Prior Art
For component digital coding of a video signal in digital TV studios, there has been proposed a method which uses the sampling frequency of 13.5 MHz for the luminance signal Y and the sampling frequency of 6.75 MHz for each of the color difference signals C.sub.R (=R-Y) and C.sub.B (=B-Y), i.e., the so-called 4:2:2 component coding system.
The sampling structure of this system is understood to be in spatially static, perpendicularly crossing lattice form (orthogonal) as shown in FIG. 4 and in which sampling positions of the Y signal represented by black dots and those of the C.sub.R and C.sub.B signals represented by white dots are shown to be in common, or coincide. Quantization is carried out in accordance with the 8-bit linear quantization system (256 levels), and the quantization levels for the peak levels corresponding to each of the Y, C.sub.R and C.sub.B signals are specified as shown in FIG. 5. Thus, the black level for the Y signal is a quantization level of 16 and the white peak level thereof is a quantization level of 235, and the maximum color difference levels for each of the C.sub.R and C.sub.B signals are quantization levels of 16 and 240.
The Y, C.sub.R and C.sub.B signals are in the following relationship with the three primary color signals R, G, and B: ##EQU1##
Conversely, the R, G, and B signals can be expressed in terms of the Y, C.sub.R and C.sub.B signals as shown in the following equations (2): ##EQU2##
In the prior art digital VTRs of the 4:2:2 component system, the Y, C.sub.R and C.sub.B signals are recorded and reproduced so as to satisfy the above mentioned conditions.
More particularly, as shown on FIG. 6, in a known digital VTR, an R signal converted into 8-bit digital data, for example, is supplied from an input 31 to a matrix circuit 34, a G signal converted into 8-bit digital data, for example, is supplied from an input 32 to the matrix circuit 34, and a B signal converted into 8-bit digital data, for example, is supplied from an input 33 to the matrix circuit 34.
In the matrix circuit 34, the R, G, and B signals are matrixed in accordance with the above equations (1) into Y, C.sub.R, and C.sub.B signals and these signals are supplied or input to a record/playback circuit 35.
In the recording side of the record/playback circuit 35, the data are encoded with error correcting codes and then recorded on a recording medium as the Y, C.sub.R, and C.sub.B component signal data. In the playback side of the record/playback circuit 35, the data recorded on the recording medium are reproduced, the reproduced Y, C.sub.R, and C.sub.B signals are subjected to an error correction process, the data which are uncorrectable in the error correction process are subjected to an error concealment process, and the thus provided output is supplied to a dematrix circuit 36.
In the dematrix circuit 36, the reproduced Y, C.sub.R, and C.sub.B signals are matrixed in accordance with the above mentioned equations (2) and the reproduced R, G, B signals are generated and output from circuit 36.
FIGS. 7A and 7B indicate the relationship between the signals that are converted through the R, G, and B signals.fwdarw.Y, C.sub.R, and C.sub.B signals conversion and the signals resulting from the reproduced Y, C.sub.R, and C.sub.B signals.fwdarw.playback R, G, and B signals conversion performed in the matrix circuit 34 and the dematrix circuit 36, respectively.
FIG. 7A expresses the R, G, and B signals in a cubic manner assuming that the amplitude of each signal is a value between 0 and 1 and that the R signal is shown in the z axis, the G signal in the y axis, and the B signal in the x axis.
The cube in FIG. 7A is transformed into the cube indicated in FIG. 7B through mapping of one-to-one correspondance performed in the matrix circuit 34 in accordance with the equations (1) defining the transformation. In FIG. 7B, the Y signal is represented by the y axis, the C.sub.B signal is represented by the z axis, and the C.sub.R signal is represented by the x axis. The transformation is as follows: ##EQU3##
FIG. 8A is a projection drawing of the cube in FIG. 7B as viewed on the (Y-C.sub.R) plane, FIG. 8B is a projection of the cube in FIG. 7B as viewed on the (Y-C.sub.B) plane, and FIG. 8C is a projection of the cube in FIG. 7B as viewed on the (C.sub.R -C.sub.B) plane.
As is apparent from FIGS. 8A, 8B, and 8C, when the amplitudes of the R, G, and B signals are assumed to be values between 0 and 1, the domains of the values of the corresponding signals Y, C.sub.R, and C.sub.B are given by the following equations (3): ##EQU4##
Further, the cube indicated in FIG. 7B is transformed again into the cube indicated in FIG. 7A through mapping of one-to-one correspondence performed in the dematrix circuit 34 in accordance with the equations (2) defining the transformation.
In the digital VTRs employing the 4:2:2 component system as described above, the input R, G, and B signal data having amplitude values between 0 and 1 independently of each other are converted by matrixing into the corresponding Y, C.sub.R, and C.sub.B signals, which are then provided with error correcting codes and recorded. In the playback side, error correction is effected for the signals and if there are some uncorrectable residual errors, these are compensated for by error concealment.
When considering the range (domain) of values that the Y, C.sub.R, and C.sub.B signals can take, it is to be noted that these values cannot be of arbitrary magnitude and independent of one another but are related to each other and subject to some restriction predetermined by their relationship with the R, G, and B signals. However, in the prior art digital VTRs, the Y, C.sub.R, and C.sub.B signals were treated as independent variables having the domains indicated in the above mentioned equations (3). Thus, for example, if the Y signal obtained from the reproduced Y, C.sub.R, and C.sub.B signals was correct and Y=1, then C.sub.R =0 and C.sub.b =0 should be obtained as a necessary consequence. However sometimes happens that such erroneous data as C.sub.R =0.5 and C.sub.B =0.5 is obtained when some error to occurs in the course of the record--playback process. If such Y, C.sub.R, and C.sub.B signals were converted by the dematrix circuit 36 of the prior art digital VTR into R, G, and B signals, their values would become R=1.5, G=0.64, and B=1.5. Thus, reproduced R, G, and B signals that could not fundamentally exist were output from circuit 36, and as a result, an abnormally bright red or blue color, for example, was produced and the quality of the picture was thereby greatly deteriorated. Even if the amplitudes of the R, G, and B signals were forcibly limited to within the range between 0 and 1, it was not possible to modify the error data so as to provide them with correct values. Moreover, once the conversion into the R, G, and B signals was made, it was difficult to modify the signals so as to restore them to their correct values since the error information for the data had already been lost.